Thermally actuated electric switch



July 28, 1953 s. M. DE PUY 2,647,189

THERMALLY ACTUATED ELECTRIC SWITCH Filed Nov. 29, 1951 Ihventpr: SEHTT'IOUT M. DePu H,

His Attorne g.

Patented July 28, 1 953 THERMALLY ACTUATED- ELECTRIC SWITCH Seymour M. De Puy, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York pplic n Nov mber 2 19-51., Se a1No.-.2., .8,825

9 Claims. 1

This invention relates to snap action devices, and more particularly to thermally-actuated, snap action electric switches.

The object of my invention, in general, is to provide a snap action device which is small and compact, and yet accurate and reliable in operation.

Another object of the invention is the provision of a small and compact, thermally-actuated electric switch which operates reliably and positively at a predetermined temperature without chatt ir or vibration of the contacts.

A still further object of this invention is to provide a small, compact, snap action electric switch which provides a relatively large gap between the fixed and movable contacts in the open position.

Still another object of my invention is to provide a small thermally-actuated electric switch which i inexpensive to manufacture.

These and other objects and advantages of my invention will be apparent from the subsequent detailed description of a preferred embodiment thereof, while the scope of the invention is set forth in the appended claims.

In carrying out my invention in one preferred form thereof, I provide a bimetal plate in the form of a cardioid, having a centrally disposed aperture separating the bimetal member into a first portion adjacent the lobes of the cardioid and a second portion relatively distant from these lobes. An indentation is formed in this second portion, for stressing the bimetal plate into a dished formation. The cardioid member is formed so that the metal having the greater temperature coefficient of expansion is on the concave surface. Means .are provided for securing the said first portion on a support member, whereupon when the temperature of the bi etal member is increased to a predetermined value, the bimetal member snaps from one position in which the metal with the greater coefiicient of expansion is on the concave surface to another position in which this surface becomes convex.

For a clearer and more complete understanding of my invention, and a better appreciation of the various advantages and features thereof, reference should be had to the following detailed description of a preferred embodiment, in connection with the accompanying drawing. In the drawing, Fig. l is a side elevational view, with the enclosure in section, of a snap action electric switch embodying my invention; Figs. 2, 3 and 3a are views of the bimetal member only of the switch of Fig. 1 with this bimetal in the unactuated position; Fig. 4 is a partial view similar to Fig. 1 except with the bimetal element in the actuated or snapped position; Fig. 5 is a view of the bimetal element only in the snapped position; while Fig. 6 shows a modified embodiment of the invention.

Referring to Fig. l of the drawing, there is shown a thermally responsive, snap action electric switch which is designated generally by the numeral Hi. This switch includes as the movable element a flatly disposed bimetal plate member II in the form of a cardioid, i. e., heart-shaped. Bimetal member l I is made of two metals having different temperature coefficients of expansion, bonded together to form a composite member having the difierent metals disposed in thin flat layers or laminations parallel to the fiat surfaces thereof. The movable element may be enclosed within an enclosure l2 of insulating material which may be cylindrical in form, having an end wall 13 closing one end of the cylinder, and a cover member I4 closing the other end of the cylinder. A support member 15 of conductive material may be installed within enclosure I2.

As shown the member (5 supports the bimetal member l I, the'latter being attached to the member l5 in a suitable manner such as by spot welding at the point IS on a projection H on the support member. To provide means for closing and opening anelectric circuit, the switch It) is provided with a movable contact 18 mounted on the lower portion of bimetal member H and a fixed contact l9 mounted on end portion I3 of the switch enclosure. Electrical connections 26 and 2i respectively may be provided for the movable and fixed contacts.

As shown in greater detail in Figs. 2, 3 and 3a, the bimetal member H is in the form of a cardioid, having a central cutout portion or aperture 22 which separates the bimetal member into a first portion '23 adjacent the lobes 24 and 25 of the cardioid and a second portion 25 which is opposite the portion 23 and relatively distant from the lobes.

In order to provide the bimetal plate I l which preferably is formed as by stamping from a flat sheet of imetallic material, with the d s positive snap action characteristics, I form an indentation 21 in the lower portion 28 of the bimetal memberin a suitable manner such as by stamping it in .a press. This indentation, which fo m a p o ect on h chma n u e a fla 1 11101128 and slant n p rt ons :29 on either side. ofthe flat portion, causes the bimetal member H to assume a dished formation in which the side portionsciland :3! of the member I l are drawn together and, at the same time, held in cambe-red relation with respect to the original norm-a1 plane of member ll (see Figs. 3 and 3a., particularly the latter). This provides abnormal stresses i-n-th-e cardioid member H, particularly at'locations 3-2 and 33 adjacent the lobes of the cardioid, and causes portion 23 of the bimetal member to be maintained at an angle with the original plane of-the bimetal in the direction of the convex surface thereof. This angle is readily apparent in Fig. 1 of the drawing.

It should be noted that Fig. 3a of the drawing shows the bimetal I I from a slightly different angle than Fig. 3 which is a conventional view looking vertically upward at the bimetal in Figs. 2 and 1. In addition to tilting the bimetal slightly in Fig. 3a, th dished formation of the bimetal and the cambered relation of the side portions 39 and 3| thereof have been exaggerated somewhat in this figure in order better to illustrate the configuration of bimetal II in the unsnapped posiion.

Since the stresses created in bimetal member II by forming indentation 21 therein are concentrated at locations 32 and 33, the cross-section of member II at these locations is preferably made relatively large. The cross-section is also relatively large adjacent location I6 at which the bimetal member II is secured to the supporting member, this being desirable to provide additional strength near point I6 and also to prevent distortion of member II due to the spot welding operation, if this means of securing the bimetal member I I on the support I is utilized.

In the condition shown in Figs. 1, 2, 3 and 3a, at a normal temperature, the bimetal member II has the metal with the greater temperature coemcient of expansion on the generally concave surface, i. e., upper surface as viewed in Figs. 3

and 3a. With the bimetal member II in this condition, if the temperature of th bimetal is increased to a predetermined value, the greater expansion of the metal on the concave surface of the bimetal causes the member II to snap from the position shown in Fig. l to a second position as shown in Fig. 4 in which the previously generally concave surface of the bimetal becomes convex or, in any event, less concave than previously. This, of course, separates contacts I8 and I9. Fig. 5 shows in more detail the configuration of the bimetal member II after this snapping action has occurred. When the temperature of the bimetal member II is again reduced suniciently, it snaps back to its original position as shown in Figs. 1, 2, 3 and 3a, closing contacts I8 and I9 again.

I have found that the use of the cardioid configuration described and illustrated herein, together with the indentation for stressing the bimetal member, makes possible a thermally-actuated, snap action switch which is small and yet provides accurate and reliable action at a predetermined temperature. Furthermore, such a switch embodying such a bimetal member can be manufactured at low cost because of the small number of parts.

In spite of the low cost and the small size, however, this construction provides a switch with a relatively large contact separation in the open position. For example, in a switch in which the largest dimension of the bimetal member II between the outer edges of portions 30 and 3I was approximately th travel of movable contact I8 between closed and open positions was approximately A3. Such a relatively large travel is very desirable in an electric switch in order to assure that the current flowing through the switch will be interrupted.

In order to reduce the cost, a bimetal member for use in a switch embodying this construction is preferably formed, 1. e., provided with indentation 21, at an accurately controlled temperature.

This causes the bimetal to snap at a predetermined higher temperature without any further adjustment or calibration. It is possible, however, to vary the snapping temperature of a previously formed bimetal member II by varying the spacing between support member I5 and fixed contact I9, that is, by varying the contact pressure in the contacts closed position.

The spacing between member I5 and contact I 3 obviously can be varied by appropriate movement of either or both of these two members. If it is desired to make this spacing readily adjustable in order to provide for the calibration of the switch, an arrangement such as that illustrated in Fig. 6 of the drawing may be used. In Fig. 6 the end wall I3 of the switch enclosure is provided with a shoulder portion 34 having an aperture 35 therethrough for receiving a bolt 38 or other means for retaining an adjustable support member 36. The portion 35a of aperture 35 adjacent member 36 is made relatively large so that member 36 is deformed slightly as a nut 31 on the outsid end of bolt 38 is tightened. Bimetal member I I is secured to adjustable support member 36 at point IS in a similar manner to the previously described arrangement of Fig. 1. As nut 31 is tightened and loosened, adjustable member 36 is caused to move respectively farther away and closer to fixed contact I9, thus causing the switch to open at respectively a higher or lower temperature.

A shown in Fig. 6, movable support member 36 is provided with an adjustable screw 39 which serves as a stop for a bimetal member II in the snapped or contacts open position. By adjusting screw 39 to decrease or increase the amount of travel of the free portion of the bimetal as the switch opens, the temperature at which the switch closes again can be increased or decreased.

An important advantage of this invention is that it is possible to adjust the contact pressure characteristic as desired by varying the position of the point of support I8. Referring to Fig. 2, as point IE is moved downwardly on upper portion 23 of the bimetal member, the contact pressure characteristic becomes more positive; that is, a certain increase in the temperature of the bimetal causes a greater increase in the contact pressure up to the point at which snapping occurs if point I6 is moved toward movable contact I3. If point of securement I6 is moved away from contact I8, the pressure characteristic becomes less positive; and if point I5 is moved sumciently far away from contact I8, the pressure of contact I8 on fixed contact I9 decreases as the temperature of bimetal member I I increases, i. e., the characteristic becomes negative. At an intermediate location for point It, the contact pressure remains approximately the same as the temperature of bimetal member II increases, and this is generally the most desirable condition.

It is desirable that the area of engagement at location I6 between bimetal I! and the support be kept as small as possible, consistent with a rigid connection, in order not to interfere with the action of the bimetal, described in the preceding paragraph, in providing a selected contact pressure characteristic. As illustrated and described herein, bimetal I I is secured by spot welding to a projection. II which comprises a segment or" a sphere, however, other equivalent support arrangements may be used.

It will be readily understood that it is not necessary for the indentation 27 to be formed in exactly the manner described and illustrated herein, as long as the indentation produces a dished arrangement of the bimetal member in which the pair of side portions 39, 3| are abnormally spaced and at the same time are held in cambered relation with the normal plane of the member H. For example, the indentation 21 could be formed so that the resulting projection as seen in Fig. 3 is downward instead of upward. It is satisfactory also to form the indentation in portion 23 of the bimetal instead of in portion 26 if desired It will be understood also that a bimetal member II which responds to a decrease in temperature instead of an increase in temperature can be made by forming the member I] with the metal having the greater temperature coeflicient of expansion on the convex surface instead of on the concave surface. Such a bimetal member snaps when the temperature is reduced to a predetermined value.

It will be understood also that movable contact I8 is not essential to the operation of a switch embodying this invention, and that the lower portion 26 of the bimetal member H can itself serve as the movable contact if desired. Another modification which is within the scope of this invention is to construct a normally open switch instead of a normally closed switch as described and illustrated, which could be done by locating fixed contact H! where it would be engaged by portion 26 of member H after it has snapped instead of before it has snapped. Similarly, a single pole, double throw switch may be made by having fixed contacts at both locations.

While a bimetallic member was employed in the embodiment of the invention illustrated and described in detail herein, a multimetallic memher, i. e., a composite member comprising laminations of more than two metals, can be substituted and utilized in the same manner providing at least two of the metals have different thermal expansion characteristics.

Therefore, while modifications of my invention may be made, it should be understood that I intend to cover by the appended claims all such modifications which fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

'1. A thermally responsive snap action device comprising a flatly disposed bimetal member in the form of a cardioid, said cardioid having a centrally disposed aperture separating the bimetal member into a first portion adjacent the lobes of said cardioid and a second portion relatively distant from said lobes, an indentation formed in said bimetal member for stressing said member into a dished formation whereby one surface of said member is generally concave and the other surface is generally convex, and means for securing a point on said first portion to a support, the metal adjacent the concave surface of said bimetal member havin a different temperature coefficient of expansion than the metal adjacent the convex surface whereby said member snaps from a first position in which said one surface is generally concave to a second position in which said one surface is generally convex upon a change in the temperature of said member to a predetermined value.

2. A thermally responsive snap action device comprising a flatly disposed bimetal member in the form of a cardioid, said cardioid having a centrally disposed aperture separating the bi- .metal member into a first portion adjacent the lobes of said cardioid and a second portion relatively distant from said lobes connected:- by a pair of side portions, an indentation formed in said bimetal member for stressing said member into a dished formation in which said side portions are held in cambered relation and simultaneously held in abnormal spaced relation with each other, and means for securing a point on said first portion to a support, the metal adjacent the generally concave surface of said bimetal member having a different temperature coefficient of expansion than the metal adjacent the convex surface whereby said member snaps from a first position in which one surface is generally concave to a second position in which said one surface is generally convex upon a change in the temperature of said member to a predetermined value.

3. A thermally responsive snap action device comprising a bimetal plate member in the form of a cardioid, a centrally disposed aperture in said cardioid separating the bimetal memberinto a first portion adjacent the lobes of said cardioid and an opposite second portion relatively distant from said lobes, an indentation formed in said second portion for stressing said bimetal member into a dished formation whereby one surface of said member is generally concave and the other surface is generally convex, means for securing a point on said first portion to a support, the metal on the concave surface of said bimetal member having a diiferent temperature coefficient of expansion than the dissimilar metal on the convex surface thereof whereby said bimetal member snaps from a first position in which said one surface is generally concave to a second position in which said one surface is generally convex upon a change in the temperature of said memher to a predetermined value, and fixed contact means against which said second portion of said member reacts in one of said two positions.

4. A thermally responsive snap action switch comprising a bimetal plate member in the form of a cardioid, said cardioid having a centrally disposed aperture separating the bimetal member into a first portion adjacent the lobes of said cardioid and an opposite second portion relatively distant from said lobes connected by a pair of side portions, an indentation formed in said second portion for stressing aid bimetal member into a dished formation in which said side portions are held in cambered relation and simultaneously held in abnormal spaced relation with each other, means for securing a point on said first portion to a support, the metal on the generally concave surface of said bimetal member having a different temperature coefiicient of expansion than the metal on the convex surface whereby said member snaps from a first position in which one surface is generally concave to a second position in which said one surface is generally convex upon a change in the temperature of said member to a predetermined value, and fixed contact means against which said second portion of said member reacts in one of said two positions.

5. A thermally responsive snap action switch comprising a bimetal plate in the form of a cardioid, a centrally disposed aperture in said cardioid separating said bimetal plate into a first portion adjacent the lobes of said cardioid and an opposite second portion relatively distant from said lobes, a pair of side portions connecting said first and second portions, an indentation formed in said second portion for stressing said bimetal plate into a dished formation in which said side portions are held in cambered relation and simultaneously held in abnormal spaced relation with each other, said stressing causing said first portion to be displaced from the normal plane of the plate in the direction of the generally convex surface thereof, means for securing a point on said first portion to a support, the metal on the generally concave surface of said bimetal plate having a greater temperature coefficient of expansion than the dissimilar metal on the convex surface thereof whereby said plate snaps from a first position in which one surface is generally concave to a second position in which said one surface is generally convex upon an increase in the temperature of said plate to a predetermined value, and fixed contact means against which said second portion of said plate reacts in said one position.

6. A thermally responsive snap action electric switch comp-rising a bimetal plate in the form of a cardioid, a centrally disposed aperture in said cardioid separating said bimetal plate into a first portion adjacent the lobes of said cardioid and an opposite second ortion relatively distant from said lobes, a pair of side portions connecting said first and second portions, an indentation formed in said second portion for stressing said bimetal plate into a dished formation in which said side portions are held in cambered relation with respect to the normal plane of said plate and simultaneously held in abnormal spaced relation with each other, said indentation forming a projection on said plate in the direction of the generally concave surface thereof, said first portion being displaced from the said normal plane of the bimetal plate in the direction of the generally convex surface thereof by said stressing, means for securing a point on said first portion to a conductive support member, a movable contact member carried by said second portion of said bimetal plate, and a fixed contact member against which said movable contact member abuts when said bimetal plate is in a first position in which the metal having the greater temperature coefficient of expansion is on the concave surface thereof, said bimetal plate snapping to a second position upon an increase in the temperature thereof to a predetermined value to change the previously concave surface to a generally convex surface thereby moving said second portion and separating said movable contact member from said fixed contact member.

7. A thermally responsive snap action device comprising a bimetal plate member in the form of a cardioid, said cardioid having a centrally disposed aperture separating said bimetal plate into a first portion adjacent the lobes of said cardioid and an opposite second portion relatively distant from said lobes, an indentation formed in said bimetal plate for stressing said plate into a dished formation in which one surface of said plate is generally concave and the other surface thereof is generally convex, and securing means providing a small area of engagement for securing a point on said first portion to a support, the metal adjacent the concave surface of said bimetal plate having a different temperature coefficient of expansion than the metal adjacent the convex surface thereof whereby said plate snaps from a first position in which said one surface is generally concave to a second position in which said one surface is generally convex upon achange in the temperature of said member of said plate to a predetermined value.

8. A thermally responsive snap action device comprising a bimetal plate member in the form of a cardioid, a centrally disposed aperture in said cardioid separating the bimetal member into a first portion adjacent the lobes of said cardioid and an opposite second portion relatively distant from said lobes, an indentation formed in said second portion for stressing said bimetal member into a dished formation whereby one surface of said member is generally concave and the other surface is generally convex, the metal on the concave surface of said bimetal member having a different temperature coefiicient of expansion than the dissimilar metal on the convex surface thereof whereby said bimetal member snaps from a first position in which said one surface is generally concave to a second position in which said one surface is generally convex upon a change in the temperature of said member to a predetermined value, securing means providing a small area of engagement for securing a point on said first portion to a support, and a fixed contact means against which said second portion of said member reactions in one of said two positions with a pressure characteristic dependent upon the distance from said second portion of the said point of securement of said first portion.

9. A thermally responsive snap action electric switch comprising a bimetal plate in the form of a cardioid, a centrally disposed aperture in said cardioid separating said bimetal plate into a first portion adjacent the lobes of said cardioid and an opposite second portion relatively distant from said lobes, a pair of side portions connecting said first and second portions, an indentation formed in said second portion for stressing said bimetal plate into a dished formation in which said side portions are held in cambered relation with respect to the normal plane of said plate and simultaneously held in abnormal spaced relation with each other, said indentation forming a projection on said plate in the direction of the generall concave surface thereof, means for securing a point on said first portion to a conductive support member at a selected point on said first portion, said securing means providing a relatively small area of engagement between said first portion and said conductive support memher, a movable contact member carried by said second portion of said bimetal plate, and a fixed contact member against which said movable contact abuts when said bimetal plate is in a first position in which the metal having the greater temperature coefficient of expansion is on the concave surface thereof, said bimetal plate snapping to a second position upon an increase in the temperature thereof to a predetermined value to change the previously concave surface to a generally convex surface thereby moving said second portion and separating said movable contact member from said fixed contact member, said bimetal plate providing a contact pressure characteristic during temperature changes prior to said snapping action dependent upon the distance of said selected point from said movable contact member.

SEYMOUR M. DE PUY.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,487,683 Wilson Nov. 8, 1949 2,487,684 Smith Nov. 8, 1949 2,564,931 smith Aug. 21, 1951 

